Reflective and transparent material on poster
As you do not need anything like refractions, I don't think you need to consider the Glass shader.

In this situation, I would personally create a simple mix of Transparent and Glossy shader, and adjust the mix until you got the look that you wanted. Obviously it will be a mix of high transparency, and just a little glossy.

To get a physically accurate result, you could use the Fresnel or Layer Weight nodes to control the mix (so you get more Glossy on the glancing angle, and less Glossy when viewed straight on). Add a Math node to multiply the Fresnel and you can play around until you get the result you want.